struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};


class Solution {
public:
    void flatten(TreeNode* root) {
        if (root == nullptr) return;
        TreeNode* right = root->right;
        flatten(root->left);
        root->right = root->left;
        root->left = nullptr;
        flatten(right);
        while (root->right) root = root->right;
        root->right = right;
    }
    
};